Electric Field Control of Interfacial Ferromagnetism in CaMnO_{3}/CaRuO_{3} Heterostructures

Emergent Ferromagnetism in CaRuO3/CaMnO3 (111)-Oriented Superlattices

The boundary between CaRuO3 and CaMnO3 is an ideal test bed for emergent magnetic ground states stabilized through interfacial electron interactions. In this system, nominally antiferromagnetic and paramagnetic materials combine to yield interfacial ferromagnetism in CaMnO3 due to electron leakage across the interface. In this work, we show that the crystal symmetry at the surface is a critical factor determining the nature of the interfacial interactions. Specifically, by growing CaRuO3/CaMnO3 heterostructures along the (111) instead of the (001) crystallographic axis, we achieve a 3-fold enhancement of the magnetization and involve the CaRuO3 layers in the ferromagnetism, which now spans both constituent materials. The stabilization of a net magnetic moment in CaRuO3 through strain effects has been long-sought but never consistently achieved, and our observations demonstrate the importance of interface engineering in the development of new functional heterostructures.

Interfacial charge transfer induced antiferromagnetic metals and magnetic phase transition in (CrO2)m/(TaO2)nsuperlattices

As a class of remarkable spintronic materials, intrinsic antiferromagnetic (AFM) metals are rare. The exploration and investigation of AFM metals are still in its infancy. Based on first-principles calculations, the interface-induced magnetic phenomena in the (CrO₂)_m/(TaO₂)_nsuperlattices are investigated, and a new series of AFM metals is predicted. Under different ratios ofm:nwith varying valence states of Cr, the (CrO₂)_m/(TaO₂)_nsuperlattices exhibit three different phases, including the AFM metal, the AFM semiconductor, and the ferromagnetic (FM) metal. In the AFM semiconducting phases, theintra-CrO₂-monolayer magnetic exchange interaction is systematically discussed, corresponding tom = 1 orm = 2. Both the localization of the Cr 3 dorbitals and the crystal-field splitting are crucial for magnetic ordering in super-exchange interactions. Based on the analyses of the AFM semiconducting phases withm = 1 andm = 2, the mechanisms of AFM metallic phases with radios ofm:n<1/2and1/2<m:n<1/1are discussed in detail. Additionally, the AFM metallic superlattices can be tuned into a FM metallic phase by applying strain in thec-direction, such as a compression of 7% in the (CrO₂)₁/(TaO₂)₃superlattice, and a tensile strain of 7% in the (CrO₂)₂/(TaO₂)₃superlattice. The phase diagram of the (CrO₂)_m/(TaO₂)_nsuperlattices is obtained as a function of the layer thickness. This work provides new insights about realizing and manipulating AFM metals in artificial superlattices or heterostructures in experiments.

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient α C = Δ M Δ E   to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.

Origin of abnormal structural transformation in a (BiPb)FeO3/SrRuO3/SrTiO3 hetero-structure probed by Rutherford backscattering

Scientific efforts are growing to understand artificial BiFeO₃/SrRuO₃/SrTiO₃-heterostructures, wherein an altered environment at each interface, caused by epitaxial strains, broken symmetry, off-stoichiometry and charge transfer, can generate a rich spectrum of exotic properties. Herein, (BiPb)FeO₃/SrRuO₃/SrTiO₃-heterostructures were sputtered with various top (BiPb)FeO₃-layers at different growth temperatures (T_g). Strain relaxation at each interface changes with T_g and generates an additional peak alongside with (BiPb)FeO₃ at a high T_g of 700 °C. Rutherford backscattering (RBS) was employed to understand this unusual behavior as to whether it is a mixture of two phases, layer splitting or inter-diffusion of elements. Surprisingly, complete overlapping of random and aligned RBS spectra from the sample with T_g = 700 °C indicates the presence of a large amount of defects/distortions at the interfaces. The RBS compositional analysis gives clear evidence of Fe and Ru vacancies to an extent that the structural integrity may not be maintained. This abnormal condition can be explained by the inter-diffusion of Pb and Bi elements into whole films and even into the top layer of the SrTiO₃ substrate, which compensates for these vacancies by substitutional replacement and is responsible for the generation of the additional SrTi(BiPb)O₃—peak. Below T_c^SrRuO₃, the magnetic properties change significantly with T_g.

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO₃ films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO₃ as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.

Interfacial Control of Ferromagnetism in Ultrathin La0.67Ca0.33MnO3 Sandwiched between CaRu1-xTixO3 (x = 0-0.8) Epilayers

Controlling functionalities in oxide heterostructures remains challenging for the rather complex interfacial interactions. Here, by modifying the interface properties with chemical doping, we achieve a nontrivial control over the ferromagnetism in ultrathin La_0.67Ca_0.33MnO₃ (LCMO) layer sandwiched between CaRu_1-xTi_xO₃ [CRTO(x)] epilayers. The Ti doping suppresses the interfacial electron transfer from CRTO(x) to LCMO side; as a result, a steadily decreased Curie temperature with increasing x, from 262 K at x = 0 to 186 K at x = 0.8, is observed for the structures with LCMO fixed at 3.2 nm. Moreover, for more insulating CRTO(x ≥ 0.5), the electron confinement induces an interfacial Mn-e_g(x²-y²) orbital order in LCMO which further attenuates the ferromagnetism. Also, in order to characterize the heterointerfaces, for the first time the doping- and thickness-dependent metal-insulator transitions in CRTO(x) films are examined. Our results demonstrate that the LCMO/CRTO(x) heterostructure could be a model system for investigating the interfacial multiple interactions in correlated oxides.

Interfacial Symmetry Control of Emergent Ferromagnetism at the Nanoscale

The emergence of complex new ground states at interfaces has been identified as one of the most promising routes to highly tunable nanoscale materials. Despite recent progress, isolating and controlling the underlying mechanisms behind these emergent properties remains among the most challenging materials physics problems to date. In particular, generating ferromagnetism localized at the interface of two nonferromagnetic materials is of fundamental and technological interest. Moreover, the ability to turn the ferromagnetism on and off would shed light on the origin of such emergent phenomena and is promising for spintronic applications. We demonstrate that ferromagnetism confined within one unit cell at the interface of CaRuO3 and CaMnO3 can be switched on and off by changing the symmetry of the oxygen octahedra connectivity at the boundary. Interfaces that are symmetry-matched across the boundary exhibit interfacial CaMnO3 ferromagnetism while the ferromagnetism at symmetry-mismatched interfaces is suppressed. We attribute the suppression of ferromagnetic order to a reduction in charge transfer at symmetry-mismatched interfaces, where frustrated bonding weakens the orbital overlap. Thus, interfacial symmetry is a new route to control emergent ferromagnetism in materials such as CaMnO3 that exhibit antiferromagnetism in bulk form.

Magnetic Interactions at the Nanoscale in Trilayer Titanates

We report on the phase diagram of competing magnetic interactions at the nanoscale in engineered ultrathin trilayer heterostructures of LaTiO_{3}/SrTiO_{3}/YTiO_{3}, in which the interfacial inversion symmetry is explicitly broken. Combined atomic layer resolved scanning transmission electron microscopy with electron energy loss spectroscopy and electrical transport have confirmed the formation of a spatially separated two-dimensional electron liquid and high density two-dimensional localized magnetic moments at the LaTiO_{3}/SrTiO_{3} and SrTiO_{3}/YTiO_{3} interfaces, respectively. Resonant soft x-ray linear dichroism spectroscopy has demonstrated the presence of orbital polarization of the conductive LaTiO_{3}/SrTiO_{3} and localized SrTiO_{3}/YTiO_{3} electrons. Our results provide a route with prospects for exploring new magnetic interfaces, designing a tunable two-dimensional d-electron Kondo lattice, and potential spin Hall applications.